CIS (CMOS Image Sensors) are used in numerous applications including digital still cameras. In semiconductor technologies, image sensors are used for sensing exposed lights projected towards a semiconductor substrate. Generally, these devices utilize an array of active pixels (i.e., image sensor elements or cells) comprising photodiode and other elements (e.g., transistors) to convert images into digital data or electrical signals. CIS products generally include a pixel (or pixel array) region and a periphery region.
STI (shallow trench isolation) is an integrated circuit feature that prevents current leakage between adjacent semiconductor components. Various approaches use a single STI structure for both the pixel array and periphery regions in CIS. That is, the depth of the STI in the pixel array region and that of the periphery region is at the same level.
Dark current is unwanted current generated by pixels in the absence of illumination. Signals related to dark current may be referred to as dark signals. Sources of dark current include impurities in the silicon wafer that can cause damages to the silicon crystal lattice by processing techniques and heat buildup in the pixel region. Excessive dark current may cause leakage and lead to image degradation and poor device performance. As pixels scale down (e.g., in advanced CIS), dark current leakage tolerance also needs to be scaled down.